The present invention relates to a microwave powered lamp, and particularly to such a lamp which has a compact structure.
Recently, microwave powered lamps utilizing sulfur or selenium based fills for efficiently radiating in the visible region have been disclosed. For example, see U.S. Pat. No. 5,404,076, issued Apr. 4, 1995, which is incorporated herein by reference.
Such microwave lamps may be used as illumination sources, which find a particular use in commercial or industrial lighting. For such lighting applications, it is desirable to build a lamp system which fits within the general outline of some of the lamps already in existence. Many of these are equipped with rather large inductive ballasts which are installed in overhead locations adjoining the associated lamp. Accordingly, a new lamp system will have greater utility if it occupies a package of comparable size which can be similarly placed. This requires the various parts of the electrodeless lamp system to be kept as small as reasonably possible. These parts include a quartz bulb to contain the arc plasma housed within a microwave cavity having a metal mesh to contain the microwaves but allow the escape of light, a magnetron to produce the microwaves, a waveguide to carry the microwaves from the magnetron to the cavity, a power supply to drive the magnetron and cooling fans or other means to cool the magnetron and its power supply. The lamp bulb is rotated within the microwave cavity to stabilize the discharge which adds a motor to the system as well.
To increase the versatility of the new lamp and, therefore, the number of sites in which it can be used, the lamp itself does not include a reflector. Rather the lamp is to be inserted through a hole in reflectors of several designs, suitable for use in applications requiring light dispersal over different areas. This requires the light source to extend outward from the lamp case a distance of at least 100 mm. Keeping the entry hole to a small diameter increases the efficiency of the reflector.
It is desirable to keep the overall length of the lamp as small as possible. Since the motor which rotates the bulb must be placed outside of microwave fields, it potentially adds length to the lamp system. In one such configuration, the bulb stem is fed through the coupling slot and the waveguide, and the motor and coupler are located on the other side of the waveguide, resulting in a very long stem which is subject to breakage.
A further problem is encountered in that the waveguide must have a sufficiently narrow width so that the cutoff frequency is high enough to eliminate spurious interference signals from being generated, but must have a height sufficient to prevent arcing at the location of the magnetron antenna. A conventional WR-284 waveguide is narrow enough to eliminate interference signals, but because of its height which correlates to its width in a conventional ratio of about 1 to 2, arcing results.